1. Field of the Invention
The present invention relates to an optical sheet suitably used in displays such as LCD, EL and FED and particularly having excellent effects on prevention of luminance irregularity, improvement in contrast and widening of angle of field.
2. Description of the Background Art
Displays such as LCD, EL and FED have been remarkably developed in recent years. In particular, LCD is spread in all fields such as note type personal computers and portable terminals and expected for the future. This LCD is classified into reflection type and transmission type by how to take light for illuminating a liquid crystal panel thereinto. The reflection type is a system that a reflection plate, on which an aluminum film, silver or the like having a high reflectivity has been stuck, is arranged on the back surface of a liquid crystal panel, and light which is struck on it from the surface side of the display is reflected by the reflection plate to illuminate the liquid crystal display, thereby obtaining a liquid crystal image. On the other hand, the transmission type is a system that the liquid crystal panel is illuminated by a back light unit arranged on the back surface of the liquid crystal panel. In order to prevent contrast from being deteriorated by the ground color of the aluminum or silver in the LCD of the reflection type, it is conducted to interpose a medium that moderately diffuses light between the liquid crystal panel and the reflection plate to bring the background color near to a paper white color. In order to prevent visibility from being deteriorated by a light-scattering printing pattern formed on an acrylic light guide plate making up the back light unit in the transmission type LCD on the other hand, it is so constructed that a medium that moderately diffuses light is interposed between the liquid crystal panel and the back light unit to illuminate the liquid crystal panel by uniform surface light.
As described above, the light diffusing medium (hereinafter referred to as xe2x80x9clight diffuserxe2x80x9d) is generally used in both systems of the reflection type and transmission type. As such light diffusers, are known an interior light diffuser that fine particles are dispersed in a binder resin to diffuse light in the interior of a layer, an exterior light diffuser that the surface of a resin layer is roughened for form irregularities in the surface, thereby diffuse light on the surface, and an interior-exterior light diffuser that light is diffused both in the interior and on the exterior. Japanese Patent Application No. 350446/1998 proposes that among those corresponding to the interior exterior light diffuser, a light diffuser, in which spherical fine particles are arranged in a state of a monoparticle layer on the surface of a resin layer, is lessened in loss of transmitted light by backward diffusion to achieve high transmittance and diffusiveness.
However, the light diffuser with the spherical fine particles arranged in the state of a monoparticle layer on the surface has involved a problem that its optical properties are gradually changed under high-temperature and high humidity conditions, and its blocking with another member occurs, thereby causing uneven light scattering, color irregularity and the like because a pressure sensitive adhesive is used as a binder resin. Moreover, the control of optical properties such as light transmittance, diffusiveness and haze is conducted by the height of the spherical fine particles projecting from the binder layer. The pressure sensitive adhesives forming the binder layer flows out at a high temperature or with time, and so stable optical properties have not been achieved, and the control has been difficult.
The present invention has been made with the foregoing circumstances in view. Namely, it is an object of the present invention to provide an optical sheet having a high light transmittance and good diffusiveness and possessing excellent durability that such optical properties can be retained for a long period of time even under high-temperature and high-humidity conditions.
Another object of the present invention is to provide an optical sheet raising no blocking and a production process thereof.
The present inventors have carried out an extensive investigation as to improvement in the stability to optical properties. As a result, it has been found that the optical properties are changed under high-temperature and high-humidity because spherical fine particles undergo a change, for example, sinking of the fine particles in a direction of a substrate due to the flow of a pressure sensitive adhesive, and that blocking occurs because a pressure sensitive adhesive used in a binder layer comes into contact with another member. The present inventors have also found that spherical fine particles are bonded directly or through a very thin binder to a substrate in a state of a monoparticle layer, thereby preventing the spherical fine particles from sinking into the binder to provide an optical sheet having high light transmittance and diffusiveness and also having high stability to optical properties. The present invention has been led to completion on the basis of these findings.
According to a first aspect of the present invention, there is thus provided an optical sheet comprising a transparent substrate and a binder layer laminated directly or through another layer on the transparent substrate, wherein spherical fine particles having a particle diameter distribution of 0.8 to 1.0 are embedded in a state of a monoparticle layer in the binder layer so as to partly project from the surface of the binder layer, and a distance from the top of the projected fine particles to the surface of the transparent substrate is from 100% to 110% of the volume average particle diameter of the spherical fine particles.
In this aspect, a protective layer or a layer capable of improving light diffusiveness may be provided on the surface of the binder layer in which the spherical fine particles have been embedded.
According to a second aspect of the present invention, there is also provided an optical sheet comprising a transparent substrate, and a binder layer and a protective layer successively laminated on the substrate, wherein spherical fine particles are embedded in the protective layer in such a manner that at least a part of each spherical fine particle comes into contact with the binder layer in a state of a monoparticle layer, and a part of each spherical fine particle projects from the surface of the protective layer to form a projected portion or form a projected portion in a state covered with the protective layer.
As described above, the protective layer is provided in such a manner that the spherical fine particles are projected from the surface of the protective layer in the state that the spherical fine particles are embedded therein, or projected portions are formed by the fine particles on the surface of the protective layer, whereby an optical sheet, which has such excellent stability that the optical properties thereof do not vary according to environmental conditions and the like, causes no blocking and permits easily controlling haze to a desired value, can be provided.
A first aspect of the process for producing the optical sheet of the second aspect according to the present invention comprises the steps of forming a binder layer composed of a material having stickiness on a transparent substrate, attaching spherical fine particles in a state of a monoparticle layer on the surface of the binder layer, and providing a protective layer on the binder layer, on which the spherical fine particles have been attached, in such a manner that projected portions by the spherical fine particles are formed on the surface thereof.
A second aspect of the process for producing the optical sheet of the second aspect according to the present invention comprises the steps of forming a binder layer composed of a material having stickiness on a transparent substrate, providing a protective layer on the binder layer, attaching spherical fine particles on the surface of the binder layer to form a monoparticle layer, and embedding the spherical fine particles in the protective layer by a pressing medium in such a manner that at least a part of each spherical fine particle comes into contact with the binder layer, and a part of each spherical fine particle projects from the surface of the protective layer.